Drosophila serpin 27A is a likely target for immune suppression of the blood cell-mediated melanotic encapsulation response

The Lifespan of D. melanogaster Depends on the Function of the Gagr Gene, a Domesticated gag Gene of Drosophila LTR Retrotransposons

(1) Background: The Gagr gene in Drosophila melanogaster's genome originated from the molecular domestication of retrotransposons and retroviruses' gag gene. In all Drosophila species, the Gagr protein homologs exhibit a conserved structure, indicative of a vital role. Previous studies have suggested a potential link between the Gagr gene function and stress responses. (2) Methods: We compared flies with Gagr gene knockdown in all tissues to control flies in physiological tests and RNA-sequencing experiments. (3) Results: Flies with the Gagr gene knockdown exhibited shorter lifespans compared to control flies. Transcriptome analysis revealed that Gagr knockdown flies showed elevated transcription levels of immune response genes. We used ammonium persulfate, a potent stress inducer, to elicit a stress response. In control flies, ammonium persulfate activated the Toll, JAK/STAT, and JNK/MAPK signaling pathways. In contrast, flies with the Gagr gene knockdown displayed reduced expression of stress response genes. Gene ontology enrichment analysis identified categories of genes upregulated under ammonium persulfate stress in control flies but not in Gagr knockdown flies. These genes are involved in developmental control, morphogenesis, and central nervous system function. (4) Conclusion: Our findings indicate the significance of the Gagr gene in maintaining immune response and homeostasis.

Detection and substrate portrayal on the serum phenoloxidase activity from the grub of rhinoceros beetle, Oryctes rhinoceros

Phenoloxidase (PO) is a significant biomolecule involved in humoral defence mechanism of invertebrates. Spontaneous melanization of insect haemolymph is the major hinderance for studying PO activity, as haemolymph was collected devoid of phenylthiourea. In the study, no visible melanization was observed in crude serum from the grub of Oryctes rhinoceros up to 30 min of incubation amongst crude haemolymph, diluted haemolymph, crude serum and diluted serum that were subjected to visual observation for spontaneous melanization reaction. Accordingly, crude serum was taken for evaluating PO activity. At the same time, as PO substrates tend to auto-oxidize and provide false optical density value, tris-buffered saline devoid of any substrates were used as blank for PO assays. The ideal wavelength at which maximum PO activity occurred for each substrate, namely, tyrosine, tyramine, dopamine, L-dopa, DL-dopa, catechol, protocatechuic acid and pyrogallol was determined as 407, 410, 429, 465, 403, 466, 428 and 400 nm, respectively. Additionally, time course of oxidation for each phenolic substrate by the serum PO were examined and DL-dopa was identified as the specific substrate for serum PO in the grub of O. rhinoceros. Furthermore, maximum PO activity was observed at 5 min of incubation for 10 mM of DL-dopa that was considered as optimum concentration. The ideal pH and temperature for serum PO activity was observed as 7.5 and 20°C, respectively. These results suggested that standardizing a suitable substrate is an essential prerequisite to evaluate the real PO activity of serum which might significantly fluctuate in each insect model.

Cellular and humoral immune interactions between Drosophila and its parasitoids

The immune interactions occurring between parasitoids and their host insects, especially in Drosophila-wasp models, have long been the research focus of insect immunology and parasitology. Parasitoid infestation in Drosophila is counteracted by its multiple natural immune defense systems, which include cellular and humoral immunity. Occurring in the hemocoel, cellular immune responses involve the proliferation, differentiation, migration and spreading of host hemocytes and parasitoid encapsulation by them. Contrastingly, humoral immune responses rely more heavily on melanization and on the Toll, Imd and Jak/Stat immune pathways associated with antimicrobial peptides along with stress factors. On the wasps' side, successful development is achieved by introducing various virulence factors to counteract immune responses of Drosophila. Some or all of these factors manipulate the host's immunity for successful parasitism. Here we review current knowledge of the cellular and humoral immune interactions between Drosophila and its parasitoids, focusing on the defense mechanisms used by Drosophila and the strategies evolved by parasitic wasps to outwit it.

Molecular Identification and Immunity Functional Characterization of Lmserpin1 in Locusta migratoria manilensis

Simple Summary

Insect serpins play a vital role in the defense mechanism of insects, especially in the Toll pathway and PPO (prophenoloxidase) cascade. In this study, we provided an answer to the open question of whether serpin1 was involved in the humoral immune responses of Locusta migratoria manilensis. We identified a new Lmserpin1 gene from L. migratoria manilensis and investigated its expression profiles in all examined stages and tissues. Meanwhile, by interfering with Lmserpin1 gene, we examined the mortality of L. migratoria manilensis under Metarhizium anisopliae infection, as well as the activities of protective enzymes and detoxifying enzymes and the expression level of three immune-related genes (PPAE (prophenoloxidase-activating enzyme), PPO, and defensin). The results indicated that Lmserpin1 gene up-regulated the immune responses of L. migratoria manilensis and inhibited the infection of M. anisopliae. Our results are of great importance for better understanding of the mechanism characterization of Lmserpin1 in L. migratoria manilensis.

Abstract

Serine protease inhibitors (Serpins) are a broadly distributed superfamily of proteins that exist in organisms with the role of immune responses. Lmserpin1 gene was cloned firstly from Locusta migratoria manilensis and then was detected in all tested stages from eggs to adults and six different tissues through qRT-PCR analysis. The expression was significantly higher in the 3rd instars and within integument. After RNAi treatment, the expression of Lmserpin1 was significantly down-regulated at four different time points. Moreover, it dropped significantly in the fat body and hemolymph at 24 h after treatment. The bioassay results indicated that the mortality of L. migratoria manilensis treated with dsSerpin1 + Metarhizium was significantly higher than the other three treatments. Furthermore, the immune-related genes (PPAE, PPO, and defensin) treated by dsSerpin1 + Metarhizium were significantly down-regulated compared with the Metarhizium treatment, but the activities of phenoloxidase (PO), peroxidase (POD), superoxide dismutase (SOD), glutathione S-transferase (GST), and multifunctional oxidase (MFO) were fluctuating. Our results suggest that Lmserpin1 plays a crucial role in the innate immunity of L. migratoria manilensis. Lmserpin1 probably took part in regulation of melanization and promoted the synthesis of antimicrobial peptides (AMPs).

Antifungal immune responses in mosquitoes (Diptera: Culicidae): A review

Mosquitoes transmit many parasites and pathogens to humans that cause significant morbidity and mortality. As such, we are constantly looking for new methods to reduce mosquito populations, including the use of effective biological controls. Entomopathogenic fungi are excellent candidate biocontrol agents to control mosquitoes. Understanding the complex ecological, environmental, and molecular interactions between hosts and pathogens are essential to create novel, effective and safe biocontrol agents. Understanding how mosquitoes recognize and eliminate pathogens such as entomopathogenic fungi may allow us to create insect-order specific biocontrol agents to reduce pest populations. Here we summarize the current knowledge of fungal infection, colonization, development, and replication within mosquitoes and the innate immune responses of the mosquitoes towards the fungal pathogens, emphasizing those features required for an effective mosquito biocontrol agent.

Penaeus vannamei serine proteinase inhibitor 7 (LvSerpin7) acts as an immune brake by regulating the proPO system in AHPND-affected shrimp

Acute hepatopancreatic necrosis disease (AHPND) is a recently emerged disease in aqua cultured shrimp that is caused by virulent strains of Vibrio parahaemolyticus (VP). Our previous study used transcriptomics to identify key pathogenic factors in the stomach of AHPND-infected shrimp (Litopenaeus vannamei), and here we used a different subset of the same data to construct a gene-to-gene expression correlation network to identify immune-responsive genes. LvSerpin7 was found to have the highest number of correlations after infection, and it also showed a significant increase in mRNA expression. LvSerpin7 is expressed in all tissues but its expression levels are highest in hemocytes. After successfully silencing LvSerpin7 transcript prior to AHPND challenge, mortality was significantly increased relative to the controls and reached 100% within 36 h post infection. Compared to the controls, the phenoloxidase (PO) activity also increased in both hemolymph and stomach. Recombinant LvSerpin7 inhibited shrimp PO activity in vitro, and we also found that rLvSerpin7 inhibited the growth of AHPND-causing bacteria. These results suggest that LvSerpin7 might reduce the toxic effects that result from unregulated activation of the PO defense system by AHPND-causing bacteria.

Innate humoral immune defences in mammals and insects: The same, with differences ?

The insect immune response demonstrates many similarities to the innate immune response of mammals and a wide range of insects is now employed to assess the virulence of pathogens and produce results comparable to those obtained using mammals. Many of the humoral responses in insects and mammals are similar (e.g. insect transglutaminases and human clotting factor XIIIa) however a number show distinct differences. For example in mammals, melanization plays a role in protection from solar radiation and in skin and hair pigmentation. In contrast, insect melanization acts as a defence mechanism in which the proPO system is activated upon pathogen invasion. Human and insect antimicrobial peptides share distinct structural and functional similarities, insects produce the majority of their AMPs from the fat body while mammals rely on production locally at the site of infection by epithelial/mucosal cells. Understanding the structure and function of the insect immune system and the similarities with the innate immune response of mammals will increase the attractiveness of using insects as in vivo models for studying host – pathogen interactions.

Vitamin K2 Prevents Lymphoma in Drosophila

Previous studies have established the anticancer effect of vitamin K2 (VK2). However, its effect on lymphoma induced by UBIAD1/heix mutation in Drosophila remains unknown. Therefore, we aimed to develop an in vivo model of lymphoma for the precise characterization of lymphoma phenotypes. We also aimed to improve the understanding of the mechanisms that underlie the preventative effects of VK2 on lymphoma. Our results demonstrated that VK2 prevents lymphoma by acting as an electron carrier and by correcting the function and structure of mitochondria by inhibiting mitochondrial reactive oxygen species production mtROS. Our work identifies mitochondria as a key player in cancer therapy strategies.

Rhodnius prolixus: from physiology by Wigglesworth to recent studies of immune system modulation by Trypanosoma cruzi and Trypanosoma rangeli

This review is dedicated to the memory of Professor Sir Vincent B. Wigglesworth (VW) in recognition of his many pioneering contributions to insect physiology which, even today, form the basis of modern-day research in this field. Insects not only make vital contributions to our everyday lives by their roles in pollination, balancing eco-systems and provision of honey and silk products, but they are also outstanding models for studying the pathogenicity of microorganisms and the functioning of innate immunity in humans. In this overview, the immune system of the triatomine bug, Rhodnius prolixus, is considered which is most appropriate to this dedication as this insect species was the favourite subject of VW's research. Herein are described recent developments in knowledge of the functioning of the R. prolixus immune system. Thus, the roles of the cellular defences, such as phagocytosis and nodule formation, as well as the role of eicosanoids, ecdysone, antimicrobial peptides, reactive oxygen and nitrogen radicals, and the gut microbiota in the immune response of R. prolixus are described. The details of many of these were unknown to VW although his work gives indications of his awareness of the importance to R. prolixus of cellular immunity, antibacterial activity, prophenoloxidase and the gut microbiota. This description of R. prolixus immunity forms a backdrop to studies on the interaction of the parasitic flagellates, Trypanosoma cruzi and Trypanosoma rangeli, with the host defences of this important insect vector. These parasites remarkably utilize different strategies to avoid/modulate the triatomine immune response in order to survive in the extremely hostile host environments present in the vector gut and haemocoel. Much recent information has also been gleaned on the remarkable diversity of the immune system in the R. prolixus gut and its interaction with trypanosome parasites. This new data is reviewed and gaps in our knowledge of R. prolixus immunity are identified as subjects for future endeavours. Finally, the publication of the T. cruzi, T. rangeli and R. prolixus genomes, together with the use of modern molecular techniques, should lead to the enhanced identification of the determinants of infection derived from both the vector and the parasites which, in turn, could form targets for new molecular-based control strategies.

A Venom Serpin Splicing Isoform of the Endoparasitoid Wasp Pteromalus puparum Suppresses Host Prophenoloxidase Cascade by Forming Complexes with Host Hemolymph Proteinases

To ensure successful parasitism, parasitoid wasps inject venom along with their eggs into their hosts. The venom serves to suppress host immune responses, including melanization. Venom from Pteromalus puparum, a pupal endoparasitoid, inhibits melanization of host hemolymph in vitro in a dose-dependent manner. Using assay-guided fractionation, a serpin splicing isoform with phenoloxidase inhibitory activity was identified as P puparum serpin-1, venom isoform (PpS1V). This serpin gene has 16 predicted splicing isoforms that differ only in the C-terminal region. RT-PCR results show that the specific serpin isoform is differentially expressed in the venom gland. Recombinant PpS1V (rPpS1V) suppresses host prophenoloxidase (PPO) activation rather than inhibiting the phenoloxidase directly. Pulldown assays show that PpS1V forms complexes with two host hemolymph proteins, here named Pieris rapae hemolymph proteinase 8 (PrHP8) and P. rapae prophenoloxidase-activating proteinase 1 (PrPAP1), based on gene sequence blasting and phylogenetic analysis. The role of rPrPAP1 in the PPO activation cascade and its interaction with rPpS1V were confirmed. The stoichiometry of inhibition of PrPAP1 by PpS1V is 2.3. PpS1V also inhibits PPO activation in a non-natural host, Ostrinia furnacalis, through forming a complex with O. furnacalis serine protease 13 (OfSP13), an ortholog to PrPAP1. Our results identify a venom-enriched serpin isoform in P. puparum that inhibits host PPO activation, probably by forming a complex with host hemolymph proteinase PrPAP1.

Identification and functional characterizations of serpin8, a potential prophenoloxidase-activating protease inhibitor in Pacific white shrimp, Litopenaeus vannamei

Serpins have been characterized from varieties of organisms by their inhibitory roles on serine or cysteine proteases. However, research for the functional study of serpins in crustacean is relatively small. To fully clarify the immune characterizations of serpin, a novel serpin (named Lvserpin8) encoding 414 amino acids with a 19-amino acid signal peptide and a serpin domain was identified from the Pacific white shrimp Litopenaeus vannamei. Sequence analysis indicated that the genomic Lvserpin8 gene contains 5 exons and 4 introns. The P1 residues of the predicted scissile bond in the reactive center loop (RCL) region represented for Lysine (Lys), which is in accordance with Pmserpin8, Dmserpin27A, Ofserpin3, Bmserpin3 and Msserpin3. Quantitative results showed that high mRNA expression of Lvserpin8 was detected in hepatopancreas and testis. Notably, a significant increase of Lvserpin8 was appeared post injection of Vibrio anguillarum, and Micrococcus lysodeikticus. Moreover, Lvserpin8 was knocked down in vivo by double-stranded RNA (dsRNA) mediated RNA interference (RNAi). Suppression of Lvserpin8 led to a significant increase in the transcripts of LvPPAE2 (Prophenoloxidase-activating Enzyme 2) and cumulative mortality. What's more, recombinant Lvserpin8 protein (rLvserpin8) displayed inhibition roles on trypsin activity, and prophenoloxidase activation. Taken together, the results implied that Lvserpin8 may be involved in shrimp innate immunity via the inhibition of prophenoloxidase-activating proteases.

Annotation of the Asian Citrus Psyllid Genome Reveals a Reduced Innate Immune System

Citrus production worldwide is currently facing significant losses due to citrus greening disease, also known as Huanglongbing. The citrus greening bacteria, Candidatus Liberibacter asiaticus (CLas), is a persistent propagative pathogen transmitted by the Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae). Hemipterans characterized to date lack a number of insect immune genes, including those associated with the Imd pathway targeting Gram-negative bacteria. The D. citri draft genome was used to characterize the immune defense genes present in D. citri. Predicted mRNAs identified by screening the published D. citri annotated draft genome were manually searched using a custom database of immune genes from previously annotated insect genomes. Toll and JAK/STAT pathways, general defense genes Dual oxidase, Nitric oxide synthase, prophenoloxidase, and cellular immune defense genes were present in D. citri. In contrast, D. citri lacked genes for the Imd pathway, most antimicrobial peptides, 1,3-β-glucan recognition proteins (GNBPs), and complete peptidoglycan recognition proteins. These data suggest that D. citri has a reduced immune capability similar to that observed in A. pisum, P. humanus, and R. prolixus. The absence of immune system genes from the D. citri genome may facilitate CLas infections, and is possibly compensated for by their relationship with their microbial endosymbionts.

Serpins in arthropod biology

Serpins are the largest known family of serine proteinase inhibitors and perform a variety of physiological functions in arthropods. Herein, we review the field of serpins in arthropod biology, providing an overview of current knowledge and topics of interest. Serpins regulate insect innate immunity via inhibition of serine proteinase cascades that initiate immune responses such as melanization and antimicrobial peptide production. In addition, several serpins with anti-pathogen activity are expressed as acute-phase serpins in insects upon infection. Parasitoid wasps can downregulate host serpin expression to modulate the host immune system. In addition, examples of serpin activity in development and reproduction in Drosophila have also been discovered. Serpins also function in host-pathogen interactions beyond immunity as constituents of venom in parasitoid wasps and saliva of blood-feeding ticks and mosquitoes. These serpins have distinct effects on immunosuppression and anticoagulation and are of interest for vaccine development. Lastly, the known structures of arthropod serpins are discussed, which represent the serpin inhibitory mechanism and provide a detailed overview of the process.

Serpin-5 regulates prophenoloxidase activation and antimicrobial peptide pathways in the silkworm, Bombyx mori

The prophenoloxidase (PPO) activation pathway and Toll pathway are two critical insect immune responses against microbial infection. Activation of these pathways is mediated by an extracellular serine protease cascade, which is negatively regulated by serpins. In this study, we found that the mRNA abundance of silkworm serpin-5 (BmSpn-5) increased dramatically in the fat body after bacterial infection. The expression level of antimicrobial peptides (AMPs), gloverin-3, cecropin-D and -E decreased in the silkworm larvae injected with recombinant BmSpn-5 protein. Meanwhile, the inhibition of beads melanization, systemic melanization and PPO activation by BmSpn-5 was also observed. By means of immunoaffinity purification and analysis by mass spectrometry, we identified that the silkworm clip domain serine proteases BmHP6 and BmSP21 form a complex with BmSpn-5, which suggests that BmHP6 and SP21 are the cognate proteases of BmSpn-5 and are essential in the serine protease cascade that activates the Toll and PPO pathways. Our study provides a comprehensive characterization of BmSpn-5 and sheds light on the multiple pathways leading to PPO activation and their regulation by serpins.

Recombinant expression and characterization of a serine protease inhibitor (Lvserpin7) from the Pacific white shrimp, Litopenaeus vannamei

Serine protease inhibitors (serpins) are widely known to its inhibitory role on proteases involved in the immune responses. Herein, a novel serine protease inhibitor (Lvserpin7), encoding for 411 amino acids with calculated molecular mass of 46.29 kDa and isoelectric point of 6.98 was characterized from the Pacific white shrimp Litopenaeus vannamei. Lvserpin7 shared 92.9% identities to Penaeus monodon serpin7. Among the tested tissues, Lvserpin7 was mainly expressed in hemocytes and gill. The expression profiles analysis indicated that Lvserpin7 was significantly up-regulated in the early stage upon Vibrio anguillarum, Micrococcus lysodeikticus or White Spot Syndrome Virus (WSSV) infection. Fusion protein expression was induced by IPTG, and the purified recombinant Lvserpin7 protein (rLvserpin7) binds to both the Gram-positive and Gram-negative bacteria. Also rLvserpin7 exhibited inhibitory activity against the proteases secreted by Bacillus subtilis. Moreover, rLvserpin7 showed inhibition role on prophenoloxidase activation. To recap, we proposed that Lvserpin7 was implicated in the shrimp immunity via the inhibition of bacterial proteases and proteases involved in prophenoloxidase system.

PtSerpin from the swimming crab Portunus trituberculatus, a putative regulator of prophenoloxidase activation with antibacterial activity

Serpin or serine protease inhibitor is the largest family of protease inhibitors involved in many innate immune pathways, particularly the prophenoloxidase (proPO) activating system in arthropod. Here, we report the molecular and functional characterization of PtSerpin identified from the swimming crab Portunus trituberculatus. The genomic sequence encoding mature peptide of PtSerpin gene contained two exons of 84 and 1098 bp separated by one intron of 111 bp. The recombinant PtSerpin (rPtSerpin) with a predicted size of 44 kDa was expressed in Escherichia coli system, purified and assayed for its activities. The rPtSerpin exhibited inhibitory activity against trypsin in a dose-dependent manner, but did not affect chymotrypsin, which could define a role for PtSerpin as a trypsin inhibitor. The rPtSerpin could inhibit the growth of Gram-negative bacterium Vibrio alginolyticus, but not the tested Gram-positive bacterium and fungus. Further phenoloxidase (PO) assay showed PO activity was dramatically increased in hemocyte lysate supernatant of P. trituberculatus upon bacterial challenge. The rPtSerpin could depress the crab proPO system activation in vitro, and it could lead to 100% inhibition of PO activity under the concentration of 8.62 μM. Moreover, the rPtSerpin was able to inhibit the PO activity induced by rPtcSP and rPtSPH1. These results together indicate that PtSerpin is a potential trypsin inhibitor and may participate in crab innate immunity by the inhibition of bacterial growth and the regulation of proPO system.

Insect prophenoloxidase: the view beyond immunity

Insect prophenoloxidase (PPO) is an important innate immunity protein due to its involvement in cellular and humoral defense. It belongs to a group of type-3 copper-containing proteins that occurs in almost all organisms. Insect PPO has been studied for over a century, and the PPO activation cascade is becoming clearer. The insect PPO activation pathway incorporates several important proteins, including pattern-recognition receptors (PGRP, β GRP, and C-type lectins), serine proteases, and serine protease inhibitors (serpins). Due to their complexity, PPO activation mechanisms vary among insect species. Activated phenoloxidase (PO) oxidizes phenolic molecules to produce melanin around invading pathogens and wounds. The crystal structure of Manduca sexta PPO shows that a conserved amino acid, phenylalanine (F), can block the active site pocket. During activation, this blocker must be dislodged or even cleaved at the N-terminal sequence to expose the active site pockets and allow substrates to enter. Thanks to the crystal structure of M. sexta PPO, some domains and specific amino acids that affect PPO activities have been identified. Further studies of the relationship between PPO structure and enzyme activities will provide an opportunity to examine other type-3 copper proteins, and trace when and why their various physiological functions evolved. Recent researches show that insect PPO has a relationship with neuron activity, longevity, feces melanization (phytophagous insects) and development, which suggests that it is time for us to look back on insect PPO beyond the view of immunity in this review.

Antibacterial activity of serine protease inhibitor 1 from kuruma shrimp Marsupenaeus japonicus

Serine protease inhibitors (Serpins) are a large family of protease inhibitors involved in many critical biological processes such as blood coagulation, fibrinolysis, programmed cell death, development, and innate immunity. We identified MjSerp1, a serpin in the kuruma shrimp Marsupenaeus japonicus. The MjSerp1 cDNA has a 1239 bp open reading frame (ORF) that encodes a 412-amino acid protein with a 23 aa signal peptide and a classic serpin domain. MjSerp1 has a calculated molecular mass of 46.3 kDa and a predicted isoelectric point of 5.51. MjSerp1 is mainly expressed in the hepatopancreas and the intestines, and is moderately expressed in hemocytes. Expression pattern analysis indicated that MjSerp1 is upregulated in the hepatopancreas after Vibrio anguillarum challenge. rMjSerp1 inhibits three Gram-positive bacteria and two Gram-negative bacteria, but does not inhibit phenoloxidase activity. The microorganism binding assay showed that rMjSerp1 closely binds to both Gram-positive and Gram-negative bacteria. MjSerp1 also exhibits inhibitory activity against microbial serine proteases, such as subtilisin A and proteinase K, indicating that MjSerp1 acts as a microbial serine protease inhibitor. rMjSerp1 injection into shrimp enhances V. anguillarum clearance, but MjSerp1 knockdown through RNA interference impairs Vibrio clearance in vivo. These results indicate that MjSerp1 functions as a direct effector in the bacterial clearance of M. japonicus. All together, our findings provide novel evidences for the serine protease inhibitor in shrimp immunity.

Identification, characterization and functional analysis of a serine protease inhibitor (Lvserpin) from the Pacific white shrimp, Litopenaeus vannamei

As important arthropod immune responses, prophenoloxidase (proPO) activation and Toll pathway initiation are mediated by serine proteinase cascades and regulated by serpins. Herein, a serine protease inhibitor (Lvserpin), encoding for 415 amino acids with calculated molecular weight of 46,639 Da and isoelectric point of 7.03 was characterized from the Pacific white shrimp Litopenaeus vannamei. Multiple sequence alignment revealed that Lvserpin shared the highest similarity with Penaeus monodon serpin6 (87%). Quantitative real-time PCR (qRT-PCR) results showed that the transcripts of Lvserpin were detected in all the examined tissues and most highly expressed in gill. The expression profiles of Lvserpin were greatly fluctuated upon infection of Vibrio anguillarum, Micrococcus lysoleikticus or White Spot Syndrome Virus (WSSV). Double stranded RNA-mediated suppression of Lvserpin resulted in a significant increase in the transcripts of two clip-domain serine proteinases (PPAE and PPAF), prophenoloxidase (proPO), anti-lipopolysaccharide factor (ALF), Crustin and penaeidin3 (Pens3) and also increased the high cumulative mortality post V. anguillarum injection. Besides, the recombinant Lvserpin protein (rLvserpin) was purified and exhibited inhibitory activity against trypsin. Also the rLvserpin showed inhibition on prophenoloxidase activation and bacterial growth. Hence, we proposed that the Lvserpin played important role in the shrimp innate immunity.

Conserved microRNA miR-8 blocks activation of the Toll pathway by upregulating Serpin 27 transcripts

microRNAs (miRNAs) play significant regulatory roles in gene expression at the post-transcriptional level. This includes modulating processes such as development, immunity, cancer, and host-pathogen interactions. It was recently shown that the phylogenetically deeply conserved miRNA, miR-8, plays a role in maintaining the homeostasis of immunity by suppressing the production of anti-microbial peptides. In this study, we show that miR-8 from the insect Plutella xylostella positively regulates the transcript levels of the serine protease inhibitor Serpin 27, which has been shown to regulate activation of the Toll pathway and prophenoloxidase involved in the melanization response in insects. Interestingly, miR-8 is downregulated following parasitization by Diadegma semiclausum leading to significant declines in Serpin 27 transcript levels. This allows upregulation of antimicrobial peptides, such as gloverin, that are controlled by the Toll pathway and activation of proteolytic cascades essential for humoral immune responses to foreign invasion.

Identification and characterization of a serine protease inhibitor (PtSerpin) in the swimming crab Portunus trituberculatus

Serine protease inhibitors (Serpins) play a key role in diverse immune biological processes. A serine protease inhibitor (Serpin), namely PtSerpin, was identified from the haemocyte cDNA library of swimming crab Portunus trituberculatus. The full-length PtSerpin cDNA was 1593 bp, including an open reading frame (ORF) of 1227 bp encoding a polypeptide of 408 amino acids with estimated molecular mass of 45.048 kDa and theoretical isoelectric point of 7.23. Predicted tertiary structure of PtSerpin contained three β-sheets and nine α-helices. Multiple sequence alignment revealed that deduced amino acid sequence of PtSerpin shared the highest similarity with serpin SPI from green mud crab Scylla paramamosain (SpSerpin). Phylogenetic analysis supported PtSerpin and SpSerpin were closely related to serpins from Penaeus monodon and Daphnia pulex while other decapods formed a separate group. Although the mRNA transcripts of PtSerpin could be detected in all the examined tissues, the higher levels were present in haemocytes and gills which are the major organs respond to pathogenic microorganism. After challenged by Vibrio alginolyticus, Micrococcus luteus and Pichia pastoris, the temporal expression of PtSerpin gene in haemocytes showed different activation times against bacteria and fungi within the experimental period of 72 h. These findings suggest that PtSerpin is involved in the antibacterial defense mechanism of P. trituberculatus crab.

Dynamic coordination of innate immune signaling and insulin signaling regulates systemic responses to localized DNA damage

Metazoans adapt to changing environmental conditions and to harmful challenges by attenuating growth and metabolic activities systemically. Recent studies in mice and flies indicate that endocrine signaling interactions between insulin/IGF signaling (IIS) and innate immune signaling pathways are critical for this adaptation, yet the temporal and spatial hierarchy of these signaling events remains elusive. Here, we identify and characterize a program of signaling interactions that regulates the systemic response of the Drosophila larva to localized DNA damage. We provide evidence that epidermal DNA damage induces an innate immune response that is kept in check by systemic repression of IIS activity. IIS repression induces NFκB/Relish signaling in the fat body, which is required for recovery of IIS activity in a second phase of the systemic response to DNA damage. This systemic response to localized DNA damage thus coordinates growth and metabolic activities across tissues, ensuring growth homeostasis and survival of the animal.

Inhibition of melanization by a Nasonia defensin-like peptide: implications for host immune suppression

The parasitic wasp Nasonia vitripennis suppresses host immune mechanisms that include melanization reactions. Melanization is an important immune response of hosts induced by wasp infection and thus its inhibition represents a successful strategy for parasitism. However, the molecular basis associated with such inhibition is largely unknown in N. vitripennis. Here, we report recombinant expression, structural and functional characterization of a Nasonia-derived defensin-like peptide (called nasonin-3) whose recombinant product exerts inhibitory effect on host melanization. The possible role of nasonin-3 in immune suppression is also discussed.

Differential transcript expression between the microfilariae of the filarial nematodes, Brugia malayi and B. pahangi

Background

Brugia malayi and B. pahangi are two closely related nematodes that cause filariasis in humans and animals. However, B. pahangi microfilariae are able to develop in and be transmitted by the mosquito, Armigeres subalbatus, whereas most B. malayi are rapidly melanized and destroyed within the mosquito hemocoel. A cross-species microarray analysis employing the B. malayi V2 array was carried out to determine the transcriptional differences between B. malayi and B. pahangi microfilariae with similar age distribution.

Results

Following microarray data analysis, a list of preferentially expressed genes in both microfilariae species was generated with a false discovery rate estimate of 5% and a signal intensity ratio of 2 or higher in either species. A total of 308 probes were preferentially expressed in both species with 149 probes, representing 123 genes, in B. pahangi microfilariae and 159 probes, representing 107 genes, in B. malayi microfilariae. In B. pahangi, there were 76 (62%) up-regulated transcripts that coded for known proteins that mapped into the KEGG pathway compared to 61 (57%) transcripts in B. malayi microfilariae. The remaining 47 (38%) transcripts in B. pahangi and 46 (43%) transcripts in B. malayi microfilariae were comprised almost entirely of hypothetical genes of unknown function. Twenty-seven of the transcripts in B. pahangi microfilariae coded for proteins that associate with the secretory pathway compared to thirty-nine in B. malayi microfilariae. The data obtained from real-time PCR analysis of ten genes selected from the microarray list of preferentially expressed genes showed good concordance with the microarray data, indicating that the microarray data were reproducible.

Conclusion

In this study, we identified gene transcripts that were preferentially expressed in the microfilariae of B. pahangi and B. malayi, some of which coded for known immunomodulatory proteins. These comparative transcriptome data will be of interest to researchers keen on understanding the inherent differences, at the molecular level, between B. malayi and B. pahangi microfilariae especially because these microfilariae are capable of surviving in the same vertebrate host but elicit different immune response outcomes in the mosquito, Ar. subalbatus.

The role of melanization and cytotoxic by-products in the cellular immune responses of Drosophila against parasitic wasps

The cellular innate immune response of several species of Drosophila terminates with the encasement of large foreign objects within melanotic capsules comprised of several layers of adhering blood cells or hemocytes. This reaction is manifested by various Drosophila hosts in response to infection by endoparasitic wasps (i.e., parasitoids). Creditable assessments of the factor(s) causing, or contributing to, parasite mortality have long been considered as cytotoxic elements certain molecules associated with enzyme-mediated melanogenesis. However, observations that warrant additional or alternative considerations are those documenting parasitoid survival despite melanotic encapsulation, and those where parasitoids are destroyed with no evidence of this host response. Recent studies of the production of some reactive intermediates of oxygen and nitrogen during infection provide a basis for proposing that these molecules constitute important components of the immune arsenal of Drosophila. Studies of the virulence factors injected by female wasps during oviposition that suppress the host response will likely facilitate identification of the cytotoxic molecules as well as the cell-signaling pathways that regulate their synthesis.

Variation of Leptopilina boulardi success in Drosophila hosts: what is inside the black box?

Interactions between Drosophila hosts and parasitoid wasps are among the few examples in which occurrence of intraspecific variation of parasite success has been studied in natural populations. Such variations can originate from three categories of factors: environmental, host and parasitoid factors. Under controlled laboratory conditions, it is possible to focus on the two last categories, and, using specific reference lines, to analyze their respective importance. Parasitoid and host contributions to variations in parasite success have largely been studied in terms of evolutionary and mechanistic aspects in two Drosophila parasitoids, Asobara tabida and, in more details, in Leptopilina boulardi. This chapter focuses on the physiological and molecular aspects of L. boulardi interactions with two Drosophila host species, while most of the evolutionary hypotheses and models are presented in Chapter 11 of Dupas et al.

Cathepsin L-like cysteine proteinase (DcCathL) from Delia coarctata (wheat bulb fly): basis of insecticidal activity

A cDNA encoding a cathepsin L-like cysteine proteinase (DcCathL) was prepared from gut tissue of larvae of wheat bulb fly (Delia coarctata: Diptera). The predicted protein is a homologue of the product of Drosophila melanogaster gene Cp-1 (CG6692), and is similar to a sub-family of cysteine proteinases found in other insects which have roles in tissue remodelling during development, and moulting. Recombinant DcCathL was produced using the yeast Pichia pastoris as expression host, and showed hydrolytic activity in vitro towards the synthetic substrate Z-Phe-Arg-AMC with a pH optimum of 4.5. DcCathL was insecticidal to lepidopteran larvae when injected into haemolymph, causing mortality that was accompanied by systemic melanisation, suggesting that DcCathL was affecting the immune-related proteolytic activation cascade leading to production of active phenoloxidase. This process is normally negatively regulated by serpins in the haemolymph. Recombinant serpins from cabbage moth (Mamestra brassicae) did not inhibit DcCathL, and were susceptible to degradation by the enzyme in vitro in buffer and extracted haemolymph. When M. brassicae larvae were co-injected with a lethal dose of DcCathL and exogenous recombinant serpins, no mortality or systemic melanisation was observed, suggesting that the insecticidal effects of DcCathL in vivo result from degradation of endogenous serpins.

Ixodes scapularis tick serine proteinase inhibitor (serpin) gene family; annotation and transcriptional analysis

BACKGROUND

Serine proteinase inhibitors (Serpins) are a large superfamily of structurally related, but functionally diverse proteins that control essential proteolytic pathways in most branches of life. Given their importance in the biology of many organisms, the concept that ticks might utilize serpins to evade host defenses and immunizing against or disrupting their functions as targets for tick control is an appealing option.

RESULTS

A sequence homology search strategy has allowed us to identify at least 45 tick serpin genes in the Ixodes scapularis genome that are structurally segregated into 32 intronless and 13 intron-containing genes. Nine of the intron-containing serpins occur in a cluster of 11 genes that span 170 kb of DNA sequence. Based on consensus amino acid residues in the reactive center loop (RCL) and signal peptide scanning, 93% are putatively inhibitory while 82% are putatively extracellular. Among the 11 different amino acid residues that are predicted at the P1 sites, 16 sequences possess basic amino acid (R/K) residues. Temporal and spatial expression analyses revealed that 40 of the 45 serpins are differentially expressed in salivary glands (SG) and/or midguts (MG) of unfed and partially fed ticks. Ten of the 38 serpin genes were expressed from six to 24 hrs of feeding while six and fives genes each are predominantly or exclusively expressed in either MG and SG respectively.

CONCLUSION

Given the diversity among tick species, sizes of tick serpin families are likely to be variable. However this study provides insight on the potential sizes of serpin protein families in ticks. Ticks must overcome inflammation, complement activation and blood coagulation to complete feeding. Since these pathways are regulated by serpins that have basic residues at their P1 sites, we speculate that I. scapularis may utilize some of the serpins reported in this study to manipulate host defense. We have discussed our data in the context of advances on the molecular physiology of I. scapularis. Although the paper is descriptive, this study provides the first step toward a comprehensive understanding of serpins in tick physiology.

A serpin from the parasitoid wasp Leptopilina boulardi targets the Drosophila phenoloxidase cascade

The insect phenoloxidase (PO) cascade is known to be tightly regulated by serine proteases and serine protease inhibitors of the serpin family. As a key component of the insect immune system, it is also suspected to be inhibited by several endoparasitoid wasps, insects that develop inside other arthropods as hosts. However, the underlying mechanisms of this inhibition are largely undescribed. Here, we report the characterization of a gene encoding a serpin, LbSPNy, highly expressed in the venom of the wasp Leptopilina boulardi (IS(y) type), and we show that either the venom or the recombinant LbSPNy inhibit the PO cascade in the hemolymph of Drosophila yakuba host larva. Altogether, our results identify the first serpin used as a virulence factor by a parasitoid wasp and show that it disrupts the activation pathway of the PO in the Drosophila host.

Virulence strategies in parasitoid Hymenoptera as an example of adaptive diversity

Parasitoids are mostly insects that develop at the expense of other arthropods, which will die as a result of the interaction. Their reproductive success thus totally depends on their ability to successfully infest their host whose reproductive success relies on its own ability to avoid or overcome parasitism. Such intense selective pressures have resulted in extremely diverse adaptations in parasitoid strategies that ensure parasitism success. For instance, wasp-specific viruses (polydnaviruses) are injected into the host by parasitoid females to modulate its physiology and immunity. This article synthesizes available physiological and molecular data on parasitoid virulence strategies and discusses the evolutionary processes at work.

The proPO-system: pros and cons for its role in invertebrate immunity

Melanisation is an important immune response in many invertebrates. Recent evidence also strongly implies that the melanisation (prophenoloxidase activating) cascade is intimately associated with the appearance of factors stimulating cellular defence by aiding phagocytosis and encapsulation reactions. However, some controversy exists in the field, and at least in flies and mosquitoes, the successful combat of some pathogens does not seem to be dependent on phenoloxidase activity. This may be because of redundancy among separate immune mechanisms, inappropriate testing, species differences or a combination thereof. Recently, by using RNA interference against phenoloxidase or in specific host-pathogen interactions where the pathogen prevents melanin production by the host, convincing data have confirmed the importance of this cascade in invertebrate innate immunity.

Microarray analyses reveal distinct roles for Rel proteins in the Drosophila immune response

The NF-kappaB group of transcription factors play an important role in mediating immune responses in organisms as diverse as insects and mammals. The fruit fly Drosophila melanogaster express three closely related NF-kappaB-like transcription factors: Dorsal, Dif, and Relish. To study their roles in vivo, we used microarrays to determine the effect of null mutations in individual Rel transcription factors on larval immune gene expression. Of the 188 genes that were significantly up-regulated in wild-type larvae upon bacterial challenge, overlapping but distinct groups of genes were affected in the Rel mutants. We also ectopically expressed Dorsal or Dif and used cDNA microarrays to determine the genes that were up-regulated in the presence of these transcription factors. This expression was sufficient to drive expression of some immune genes, suggesting redundancy in the regulation of these genes. Combining this data, we also identified novel genes that may be specific targets of Dif.

Mosquito transcriptome changes and filarial worm resistance in Armigeres subalbatus

Background

Armigeres subalbatus is a natural vector of the filarial worm Brugia pahangi, but it rapidly and proficiently kills Brugia malayi microfilariae by melanotic encapsulation. Because B. malayi and B. pahangi are morphologically and biologically similar, the Armigeres-Brugia system serves as a valuable model for studying the resistance mechanisms in mosquito vectors. We have initiated transcriptome profiling studies in Ar. subalbatus to identify molecular components involved in B. malayi refractoriness.

Results

These initial studies assessed the transcriptional response of Ar. subalbatus to B. malayi at 1, 3, 6, 12, 24, 48, and 72 hrs after an infective blood feed. In this investigation, we initiated the first holistic study conducted on the anti-filarial worm immune response in order to effectively explore the functional roles of immune-response genes following a natural exposure to the parasite. Studies assessing the transcriptional response revealed the involvement of unknown and conserved unknowns, cytoskeletal and structural components, and stress and immune responsive factors. The data show that the anti-filarial worm immune response by Ar. subalbatus to be a highly complex, tissue-specific process involving varied effector responses working in concert with blood cell-mediated melanization.

Conclusion

This initial study provides a foundation and direction for future studies, which will more fully dissect the nature of the anti-filarial worm immune response in this mosquito-parasite system. The study also argues for continued studies with RNA generated from both hemocytes and whole bodies to fully expound the nature of the anti-filarial worm immune response.

Phenoloxidase Is an Important Component of the Defense against Aeromonas hydrophila Infection in a Crustacean, Pacifastacus leniusculus

The melanization cascade, in which phenoloxidase is the terminal enzyme, appears to play a key role in recognition of and defense against microbial infections in invertebrates. Here, we show that phenoloxidase activity and melanization are important for the immune defense toward a highly pathogenic bacterium, Aeromonas hydrophila, in the freshwater crayfish, Pacifastacus leniusculus. RNA interference-mediated depletion of crayfish prophenoloxidase leads to increased bacterial growth, lower phagocytosis, lower phenoloxidase activity, lower nodule formation, and higher mortality when infected with this bacterium. In contrast, if RNA interference of pacifastin, an inhibitor of the crayfish prophenoloxidase activation cascade, is performed, it results in lower bacterial growth, increased phagocytosis, increased nodule formation, higher phenoloxidase activity, and delayed mortality. Our data therefore suggest that phenoloxidase is required in crayfish defense against an infection by A. hydrophila, a highly virulent and pathogenic bacterium to crayfish.

Parasite suppression of the oxidations of eumelanin precursors in Drosophila melanogaster

In insects, eukaryotic endoparasites encounter a series of innate immune effector responses mediated in large part by circulating blood cells (hemocytes) that rapidly form multilayer capsules around foreign organisms. Critical components of the encapsulation response are chemical and enzyme-catalyzed oxidations involving phenolic and catecholic substrates that lead to synthesis of eumelanin. These responses are initiated immediately upon infection and are very site-specific, provoking no undesirable systemic responses in the host. In this study, we were interested to learn if the principal oxidation pathways leading to the synthesis of eumelanin in larvae of Drosophila melanogaster were targets for inhibition by immune suppressive factors (ISF) derived from a virulent strain of the endoparasitic wasp Leptopilina boulardi. Comparative in vitro assays monitored by sensitive electrochemical detection methods showed that ISF derived from female reproductive tissues significantly diminished the oxidations of the two diphenol eumelanin precursors, dopamine and 5,6-dihydroxyindole (DHI). The oxidations of the monophenol tyrosine, and two other related diphenols, dopa and 5,6-dihydroxyindole-2-carboxylic acid (DHICA), were not significantly inhibited by ISF. The data suggest that melanogenesis represents at least one of the host responses suppressed by L. boulardi ISF, and that the oxidation pathways selectively targeted for inhibition are those synthesizing decarboxylated pigment precursors derived from DHI. These observations, together with previous reports of adverse effects of ISF on the ability of hemocytes to adhere to foreign surfaces, suggest a multifaceted approach by the parasitoid to circumvent the innate immune response of D. melanogaster.

Tolerance to Bacillus thuringiensis endotoxin in immune-suppressed larvae of the flour moth Ephestia kuehniella

Tolerance to Bacillus thuringiensis crystal endotoxins (Bt-toxins) is correlated with an elevated immune status in larvae of the flour moth Ephestia kuehniella. To gain more specific information about the effector pathways involved in the protection against the toxin, we studied the effects of Bt-toxin formulations in susceptible (non-induced) and tolerant (immune-induced) larvae after natural (parasitism-mediated) and chemical (tropolone-mediated) suppression of defence reactions. Although melanization in hemolymph was significantly reduced, there was no significant effect on susceptibility to the toxin in parasitised or tropolone-treated larvae. This suggests that melanization of hemolymph is correlated with an elevated immune status but not responsible for the observed tolerance to Bt-toxin. To examine whether hemolymph proteins exist in the gut lumen and function as pro-coagulants, we compared gut and plasma proteins of immune-induced with those of non-induced larvae. Here we show that the lipid carrier lipophorin represents a major component in the gut lumen and interacts with mature Bt-toxin to form a complex.

Molecular and expression analysis of a family of the Amblyomma americanum tick Lospins

Serine proteinase inhibitors (serpins) are a family of structurally similar but functionally diverse proteins that regulate several important proteolytic cascades in most branches of life. We have characterized 17 Amblyomma americanum serpin cDNAs here named as `Lospins' (L; an acronym for Lone Star tick serpin) that possess three β-sheets, eight α-helices and a reactive center loop consistent with the consensus serpin superfamily secondary structures. Visual inspection of deduced amino acid sequences revealed two patterns of basic residues: (i) 86DKSRVLKAYKRL97 in L5 and L13–16 and (ii) 158VRDKTRGKI166 in all Lospins, which are similar to consensus glycosaminoglycan (GAG) binding sites (XBnXmBX, where X and B are non-basic and basic residues, n=1 or 2 and m=1, 2 or 3). On three-dimensional models, the two putative GAG binding sites mapped onto α-helices D and F, respectively, with calculation of electrostatic surface potentials revealing basic patches on L5 and L13–16 models that are comparable to the heparin-binding site on antithrombin. RT-PCR expression analysis of 15 selected genes showed that the majority (11/15) of the Lospins were ubiquitously expressed in the midgut, ovary and salivary glands. On a neighbor-joining phylogeny guide tree, 15 serpins from other ticks and 17 Lospins from this study, a total of 32 tick serpin sequences, segregated into five groups with Lospins in groups A and D being conserved across tick species. The discovery of Lospins in this study sets the framework for future studies to understand the role of serpins in tick physiology.

Laminin and a Plasmodium ookinete surface protein inhibit melanotic encapsulation of Sephadex beads in the hemocoel of mosquitoes

In refractory mosquitoes, melanotic encapsulation of Plasmodium ookinetes and oocysts is a commonly observed immune response. However, in susceptible mosquitoes, Plasmodium oocysts develop extracellularly in the body cavity without being recognized by the immune system. Like Plasmodium gallinaceum oocysts, negatively charged carboxymethyl (CM)-Sephadex beads implanted in the hemocoel of Aedes aegypti female mosquitoes were not usually melanized, but were coated with mosquito-derived laminin. Conversely, electrically neutral G-Sephadex beads were routinely melanized. Since mosquito laminin coated both CM-Sephadex beads and P. gallinaceum oocysts, we hypothesized that laminin prevents melanization of both. To test this hypothesis, we coated cyanogen-bromide-activated G-Sephadex beads with laminin, recombinant P. gallinaceum ookinete surface protein (PgS28) or bovine serum albumin (BSA). Beads were implanted into the abdominal body cavity of female Aedes aegypti and retrieved 4 days later. Uncoated controls as well as BSA-coated G-Sephadex beads were melanized in a normal manner. However, melanization of beads coated with mouse laminin, Drosophila L2-secreted proteins or PgS28 was markedly reduced. Fluorescent antibody labeling showed that PgS28-coated beads had adsorbed mosquito laminin on their surface. Thus, mosquito laminin interacting with Plasmodium surface proteins probably masks oocysts from the mosquito's immune system, thereby facilitating their development in the body cavity.

Nuclear factor-kappa B pathways in Drosophila

The nuclear factor kappa B (NF-kappaB) pathways in Drosophila are multi-component pathways, as in vertebrates, that regulate the expression of many genes responsible for the formation of dorsal-ventral polarity in the early embryo, the innate immune response to infection with Gram- negative and positive bacteria and fungi, the cellular immune response and hematopoiesis. Overactivation of the fly pathway can result in developmental defects, overproliferation of hemocytes and the formation of melanotic tumors or nodules. The extracellular events leading to the maturation of the ligand for initiation of the Drosophila NF-kappaB pathway is not conserved between flies and vertebrates, but the Toll receptor and downstream events are remarkably similar. NF-kappaB proteins have been identified in mollusks, and arthropods such as horseshoe crabs and beetles, indicating that this pathway has been established more than 500 million years ago. The fly NF-kappaB pathways are less complex than those in vertebrates, with the involvement of fewer proteins, but they are, nonetheless, just as important as their vertebrate counterparts for the life of the fly.

Melanotic mutants in Drosophila: pathways and phenotypes

Mutations in >30 genes that regulate different pathways and developmental processes are reported to cause a melanotic phenotype in larvae. The observed melanotic masses were generally linked to the hemocyte-mediated immune response. To investigate whether all black masses are associated with the cellular immune response, we characterized melanotic masses from mutants in 14 genes. We found that the melanotic masses can be subdivided into melanotic nodules engaging the hemocyte-mediated encapsulation and into melanizations that are not encapsulated by hemocytes. With rare exception, the encapsulation is carried out by lamellocytes. Encapsulated nodules are found in the hemocoel or in association with the lymph gland, while melanizations are located in the gut, salivary gland, and tracheae. In cactus mutants we found an additional kind of melanized mass containing various tissues. The development of these tissue agglomerates is dependent on the function of the dorsal gene. Our results show that the phenotype of each mutant not only reflects its connection to a particular genetic pathway but also points to the tissue-specific role of the individual gene.

Melanization immune responses in mosquito vectors

The production and deposition of melanin pigments on invading pathogens and parasites represents a unique, innate immune response in the phylum Arthropoda. This immune response has started to receive considerable attention because of the potential to exploit this mechanism to control mosquito-borne diseases. In this article, we summarize knowledge about this complex biochemistry, the use of melanin biosynthesis in diverse physiological processes and the gaps in knowledge that must be addressed if this immune process is to be manipulated in genetic-based control strategies.

Melanogenesis and associated cytotoxic reactions: applications to insect innate immunity

Insects transmit the causative agents for such debilitating diseases as malaria, lymphatic filariases, sleeping sickness, Chagas' disease, leishmaniasis, river blindness, Dengue, and yellow fever. The persistence of these diseases provides testimony to the genetic capacity of parasites to evolve strategies that ensure their successful development in two genetically diverse host species: insects and mammals. Current efforts to address the problems posed by insect-borne diseases benefit from a growing understanding of insect and mammalian immunity. Of considerable interest are recent genomic investigations that show several similarities in the innate immune effector responses and associated regulatory mechanisms manifested by insects and mammals. One notable exception, however, is the nearly universal presence of a brown-black pigment accompanying cellular innate immunity in insects. This response, which is unique to arthropods and certain other invertebrates, has focused attention on the elements involved in pigment synthesis as causing or contributing to the death of the parasite, and has even prompted speculation that the enzyme cascade mediating melanogenesis constitutes an ill-defined recognition mechanism. Experimental evidence defining the role of melanin and its precursors in insect innate immunity is severely lacking. A great deal of what is known about melanogenesis comes from studies of the process occurring in mammalian systems, where the pigment is synthesized by such diverse cells as those comprising portions of the skin, hair, inner ear, brain, and retinal epithelium. Fortunately, many of the components in the metabolic pathways leading to the formation of melanin have been found to be common to both insects and mammals. This review examines some of the factors that influence enzyme-mediated melanogenic responses, and how these responses likely contribute to blood cell-mediated, target-specific cytotoxicity in immune challenged insects.